Thanks, I think I'm covered there, the air gap on the Hammond 158M is already filled up fully with whatever varnish they use. I can't even poke a needle into it so its a solid fill. I'm not sure I understand about the Flux densities, the choke is easily 50% over the avg 50ma the Salas 6V6 draws, Salas specified 100ma minimum for the choke. I'm using 10H which is what 5U4 spec sheet suggests, all by the book. The Western Electric 274A data sheet also highly recommends choke input over capacitor input and they emphasize that.
As for the potting I'm looking at harder versions of a pourable RTV silicone, used for making deeper statue molds, the shallow molding RTV is like jello way too flexible. Also I'm going to pick up a tube of self leveling concrete crack polyurethane so I can get an idea of how hard that is when cured. I already decided I don't want rock hard epoxy, I want the choke to have some "give", like old-school tar would have. The liquid urethane may be just the ticket.
Toroidy pots the center hole of their toroid's with hard epoxy but only the center, probably to give the bolt down more strength if someone decides to torque it down like a diesel cylinder head.
In no particular order:
* in your case I would use beeswax or some other wax/paraffin (I men the waxy substance, not Brits´ name for kerosene), pick the one|with highest melting point.
* do not just "pour" it, because it will instantly harden when toucjhing cold steel/copper and not penetrate at all; you need to preheat choke first or put it in an oven or point an infrared lamp at it so wax is fluid for some time to reach everywhere. Your potting case should be closed at least on 5 faces.
Old style iron cases were so,leaving only bottom open.
* pressure mentioned associated with epoxy alludes to its significantly contracting while curing; that and the exothermal peak often destroy or damage the potted internal assembly ..... don´t ask how I know 🙁
* epoxy is irreversible, silicone too, wax and tar are not .... although tar is VERY messy.
* oil in large transformers (including now demonized PCB) was more for insulation and refrigeration than anything else.
* in your case I would use beeswax or some other wax/paraffin (I men the waxy substance, not Brits´ name for kerosene), pick the one|with highest melting point.
* do not just "pour" it, because it will instantly harden when toucjhing cold steel/copper and not penetrate at all; you need to preheat choke first or put it in an oven or point an infrared lamp at it so wax is fluid for some time to reach everywhere. Your potting case should be closed at least on 5 faces.
Old style iron cases were so,leaving only bottom open.
* pressure mentioned associated with epoxy alludes to its significantly contracting while curing; that and the exothermal peak often destroy or damage the potted internal assembly ..... don´t ask how I know 🙁
* epoxy is irreversible, silicone too, wax and tar are not .... although tar is VERY messy.
* oil in large transformers (including now demonized PCB) was more for insulation and refrigeration than anything else.
Intuitively my first thought was beeswax, high temp paraffin melts about 15-20 degrees F hotter. Reversibility of a $28 Hammond choke isn't worth the trouble, if this were a vintage might be different. OK so let's plan this... beeswax melts at 140F, not too hot at all to damage anything. So I can bolt the choke into the steel tube, cap an end, set the oven to 140F, bake it, then pour it, I can even melt the wax in the oven at the same time. Thanks for the tip on the wax solidifying on cold parts, an important step.
how about filling the can first with beeswax and the seal it off with a thinner layer of some of the other potting materials?
No need. The can is a 3.75 inch cut off length of steel rectangular tubing 2.5 x 3.5 the choke bolts to the inside of one of the small sides. I'll cap the ends with steel plate stock 2.5 x 3.5 with corners grinded radiused to the tube. So one of those caps is pre installed forming the bottom, I may weld on this end cap, but I can't weld the second cap because the unit will be wax filled. The pour will not entail much wax at all, (as the choke occupies maybe 80% of the volume), and be poured in from the other end which is really a side. Then I fabricate and install the other steel end cap, grind, sand, paint, then drill and tap the mount screw holes on whatever side I brought the leads out of. Totally steel surrounded and potted. I'm less afraid of using wax now and its lower melting point, I remember many stancor transformers and chokes in TV's I took apart as a kid that were coated in wax over the coil. My tubes will be far enough away.
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You don't want to use the hard block of tar from home depot? You just melt it outside and pour it.
I've done it this with success and it is reversible.
I've done it this with success and it is reversible.
Tar is messy compared to wax.
Try and get Polypropylene wax, it has a higher melting point compared to paraffin wax.
Take a look at the hollow core cables for telecom towers, simply support the coil inside the shield, using plastic bits.
No need really to pot, there will hardly be any vibration.
Try and get Polypropylene wax, it has a higher melting point compared to paraffin wax.
Take a look at the hollow core cables for telecom towers, simply support the coil inside the shield, using plastic bits.
No need really to pot, there will hardly be any vibration.
I disregarded tar earlier, but open to it since I'm at beeswax now modern tar is probably much less toxic than 1930's tar. And tar would have that bit of "give" like wax but a higher melting point. However there's the rub, a tar block is applied at 450F. I don't want to use that much heat.
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I found a reasonably priced seller for high temp paraffin in 10 pound blocks. Paraffin melts at 145F beeswax starts at 120F so get a few degrees over.
https://www.candlesandsupplies.com/IGI-4625-Wax-142-mp-Opaque-Pillar-per-slab
https://www.candlesandsupplies.com/IGI-4625-Wax-142-mp-Opaque-Pillar-per-slab
You could consider going to someone who does rapid prototyping and investment casting. They could inject melted wax in under vacuum into the preheated potting container for you. Sadly I am in the UK and so can't help, although I have all of the gear to do this. (There are also plenty of low viscosity polyurethane resins which mimic rubber you could also dampen vibration.)
kind regards
Marek
kind regards
Marek
Modern "Vacuum Pressure Impregnation" of coils, uses.... epoxy resin.
Not two part though. Heat cure.
Craft resin, should be capable, and casting grades are thin enough to penetrate. I've done this myself, and it works a treat.
I wouldn't use beeswax, it doesn't need to get very warm to run - wax is more suitable to something like guitar pick up coil potting, where there isn't any heat to worry about
Not two part though. Heat cure.
Craft resin, should be capable, and casting grades are thin enough to penetrate. I've done this myself, and it works a treat.
I wouldn't use beeswax, it doesn't need to get very warm to run - wax is more suitable to something like guitar pick up coil potting, where there isn't any heat to worry about
All cores must be able to freely move just a bit, due to magnetostriction. Even our DC chokes see ripple currents, so there will be forces on the core. Hard epoxy is not the best if only using that alone. You may pot in hard epoxy if you first have a thin layer of something plyable/elastic, such as a layer of silicon. The core will then have freedom to move the tiny micrometers it needs. This is mostly an issue with ferrite cores since they are brittle. Probably not an issue to worry about with iron/steel cores.
I would go the beeswax route.
I would try some sort of vacuuming as has been mentioned. It does not have to be perfect, even a vaccumcleaner can do a better job than nothing. I have a vacuum pump out of an old refrigerator. Has been providing near perfect vacuum for the 20years I've been using it (granted that is maybe once or twice a year). Plenty of diy vacuum pumps on the web... You just need a container that fits the choke, with an air tight lid, make a fitting for the tube/hose, and suck away.
I would go the beeswax route.
I would try some sort of vacuuming as has been mentioned. It does not have to be perfect, even a vaccumcleaner can do a better job than nothing. I have a vacuum pump out of an old refrigerator. Has been providing near perfect vacuum for the 20years I've been using it (granted that is maybe once or twice a year). Plenty of diy vacuum pumps on the web... You just need a container that fits the choke, with an air tight lid, make a fitting for the tube/hose, and suck away.
Correcting some misconceptions:
*We British do not call paraffin wax, kerosene or anything else.
Paraffin wax is....wax. liquid paraffin, is liquid.
Kerosene is a much lighter fraction of hydrocarbons, and used for heating and fuel.
*pressure applied to epoxy resin.
Vacuum Impregnation allows vacuum...not pressure, to draw resin into voids.
Later the reverse, pressure is applied to compress any remaining air bubbles until they are microscopic and homogeneous in their distribution.
It doesn't have anything to do with shrinkage.
Vacuum Pressure Impregnation is still the most reliable way of potting windings of all sorts, used extenisibly thoughout electrical industry.
We can be different, but it will be for the sake of being different, not any particular advantage, real nor imagined.
That might be true, but doesn't really predict how the gap will behave at 100Hz rattling. I've glued and potted over 50 different types of chokes for choke input and it might surprise you, things don't turn out as you primarily suspect.
On C-cores, I ended using only glass-hard epoxy. Even this isn't enough of a condition. You need to make sure core surfaces are flat. Unevenly cut cores, like most amorphous or nanocrystalline will buzz. My nanocrystalline and amorphous chokes slightly buzz and I can't help it. Even vacuum polyurethane potting doesn't completely make them quiet.
Wont work... Varnish wont cure into a "block" it will simply "skin over" like if you left the can open.